The present invention relates to a method for drying of lengths of carrier material, which have been printed with an ink comprising an evaporable solvent.
The present invention relates in particular to such a method, in which the lengths are guided through a chamber, a gas mixture heated by a burner is conveyed to that chamber, the gas mixture coming from such chamber is fed to the burner for heating, and a part of the gas mixture coming from the chamber is vented off.
Such a method is known from the Dutch patent application 88.00226.
The venting off of the gas mixture takes place to maintain the concentration of the solvents evaporating from the printing ink by the raised temperature beneath a certain value. Initially, this value is determined by the safety regulations, and secondly this value is determined by the fact, that the circulating gasses can of course not be saturated with solvents as otherwise no evaporation thereof can take place.
The gasses thus vented off carry a considerable amount of heat. From an energetic point of view it is thus important to keep the amount of gasses vented off as small as possible.
In such a method one aims for controlling the amount of gas mixture to be vented off such, that:
the concentration of the evaporated solvents, generally oils, is kept sufficiently beneath the value, required for safety reasons;
the concentration is kept on such a value, that printed matter of a good quality is obtained; and
the amount of gasses vented off is as small as possible.
Generally the second aim leads to a much lower value of the maximal allowable concentration, so that in practice consideration is made between the second and the third aim.
To make this consideration it is necessary to determine the concentration. It is possible to measure the concentration. The known measuring equipment used therefor is expensive and prone to faults, and it has to be calibrated regularly.
The aim of the present invention is to provide a method, in which the concentration of the evaporated solvents in the gasses is determined, and in which the disadvantages, related to direct measurement thereof, are avoided.